1. Field of the Invention
This invention relates generally to devices used for changing temperatures by either increasing the temperature or decreasing the temperature to maintain the temperature at a selected temperature level. Most specifically this invention relates to a system and apparatus for maintaining thermal equalization of the operating parts of machine tools to control thermal expansion of the various components thereof.
2. Description of the Related Art
The art is replete with systems for controlling the accuracy of machine tools by sensing temperature differentials and providing correcting factors to guide the various machine tool components to insure an accurate drilling of various work pieces. Such correction systems are typified U.S. Pat. No. 4,211,129 issued to WITTKOPP. Such systems permit the finishing of the bore of the drilling of a hole independent of the instantaneous temperature of the particular work piece. As the spindles drill a particular work piece, heat is generated through friction of the drilling action and causes not only a heating up of the work piece but also a heating of the spindle and the rotor assembly of the motor. As a result, the heat transmitted to the spindle head and generated in the spindle head by its moving parts can cause inaccuracies in position due to thermal metal expansion. The control of the temperature of the drilling head is accomplished by introducing a cooling air or fluid through the stator assembly in order to compensate for the generated heat. Such a system is disclosed in U.S. Pat. No. 4,534,686 to NAKAMURA. Other means for controlling the generated heat include inserting an insulation unit between the spindle head and spindle drive motor as described in U.S. Pat. No. 5,580,471. Of particular note is U.S. Pat. No. 5,623,857 to SAKURABA in which the temperature change of a machine tool created by a heat generating source is detected by a temperature sensor. A temperature change having substantially the same time constant as the time constant of a thermal distortion of the machine tool is calculated by using the detected temperature change. The machining error introduced by this temperature change is then positionally corrected to compensate for the thermal distortion which corresponds to the calculated temperature change. As a result, the SAKURABA patent compensates for a thermal distortion in the machine tool and thereby increase its accuracy. All of these prior art devices essentially either prevent the heat from generating, or insulate the heat from other components of the machine tool or provide compensation factors to insure that the drilling point on the physical work piece remains in the same position. Most machine tools in the prior art rely on spindle carriage of granite, ceramic or other thermal expansion resistant material to minimize spindle position changes due to thermal differences in the system. These types of carriages are expensive and include significant mass. It would be desirable to include a carriage in such systems which is made of aluminum, steel or other lightweight material and of considerable strength and lower cost. However, these metals, as well as other substitute products, are subject to significant expansion and contraction caused by temperature changes. This invention provides a thermal equalization system and apparatus which permits the use of carriages constructed from metal or other materials which are subject to thermal expansion and contraction by the control of the environment in which the elements of the machine tool operates as opposed to simply calculating displacement corrections.